<jats:p> General property of the relation between the dynamics of reduced purity and correlations in quantum mechanical systems is investigated. We show that a non-zero time-derivative of the reduced purity of a system implies the existence of non-zero correlations with its environment for any unbounded Hamiltonian with finite variance. This shows the impact of local dynamical information on the correlations, as well as the role of correlations in the mechanism of purity change. </jats:p>